Challenges in The Regulatory Use of Water Accommodated Fractions for Assessing Complex Substances

Abstract The use of the Water Accommodated Fraction (WAF) approach for the preparation of exposure systems of complex substances such as petroleum products has been a standard way to perform aquatic toxicity tests on these substances for over 30 years. In this Commentary we briefly describe the historical development, rationale, and guidance for the use and reporting of the WAF approach to assess complex substances. We then discuss two case studies, with coal tar pitch and kerosene/jet fuel, which illustrate challenges from regulatory authorities in Europe and the United States when using the WAF approach. We describe how the WAF approach is the only currently known method for testing the toxicity of the whole of a complex substance, even when some of its constituents remain unknown; it accounts for differences in the solubility of the constituents within a complex substance; and use of loading rates to describe any toxic effects is a unifying concept that allows direct comparison with releases of readily soluble substances in hazard assessment and chemical classification.